U.S. Pat. No. 6,455,648 discloses an olefin oligomerization catalyst system comprising: a chromium source, a pyrrole-containing compound, a metal alkyl, and a halide source, and a process for preparing thereof, implemented in four embodiments differing by the order of mixing the components of said system in an inert atmosphere. U.S. Pat. No. 6,455,648 also discloses a process for oligomerization of olefins by using said chromium-based catalyst system, wherein ethylene is fed at pressure of about 51 bar and temperature of 110° C. The use of the above catalyst system in the ethylene oligomerization process allows obtaining selectivity with respect to a target product of the trimerization process, hexene-1, up to 96.4%, and the selectivity with respect to hexene-1 in the hexene fraction up to 99.6%. The catalytic activity of the system is 73,400 g/(g Cr·hr) calculated on the basis of the liquid reaction products. Disadvantage of such catalyst system is formation of by-products, including decenes, in the oligomerization process, which leads to reducing the target product selectivity, and necessity of using high temperature and pressure.
U.S. Pat. No. 6,800,702 discloses the possibility of trimerization of olefins by using a highly active catalyst system formed by the chromium salt CrCl3(THF)3, a diphosphazane ligand of formula R2PN(Alk)PR2, wherein R is 2-methoxyphenyl and Alk is methyl or another alkyl, and methylalumoxane (MAO), in a Cr:ligand:MAO ratio of 1:1:300. It allows the ethylene trimerization reaction at low pressure of ethylene and simultaneous or subsequent polymerization or copolymerization of the olefin oligomer formed. The activity of the trimerization catalyst system reaches 175,300 g/(g Cr·hr) at a pressure of 8 bar. Disadvantage of said method consists in that considerable excess of the expensive MAO reagent is used for preparing said catalyst system, as well as expensive diphosphazane compounds as the components of the trimerization catalyst system. Thus, when re-calculating the outcome obtained at a pressure of 20 bar on the basis of the amount of aluminum used, the activity of the catalyst system at the pressure of 8 bar is 1,125 g/(g Al·hr).
The closest prior art for the present invention is a process of trimerization and oligomerization of olefin monomers to produce an olefin oligomer, which then can be polymerized and/or copolymerized, disclosed in patent RU 2104088. The catalyst system according to this invention consists of: a chromium salt, such as chromium (III) ethylhexanoate; a pyrrolic compound, for example 2,5-dimethylpirrole; alkyl metal, preferably, alkylaluminium, for example triethylaluminium (TEA); and, optionally, a halide source, halogen-containing compound, for example GeCl4 or AlEt2Cl. The properties of said catalyst systems are defined, firstly, by their composition and a ratio of initial components, including a solvent; secondly, by the method of mixing the initial components. The patent also discloses the possibility of an olefin trimerization process with subsequent or simultaneous copolymerization of the resultant olefin oligomer and an olefin compound. The maximum catalyst activity, according to the examples provided in the patent, is 66,400 g/(g Cr·hr) at ethylene pressure of 550 psi, or 37.4 bar, and temperature of 80° C. The Cr:Al ratio is 1:15. Thus, the specific activity of the catalyst system calculated on the basis of the amount of aluminum is 8,525 g/(g Al·hr). However, the ethylene trimerization process requires comparatively high pressure. Although the reaction is possible at significantly lower pressure down to atmospheric, it is more preferable to use the ethylene pressure not lower than 12 bar; otherwise, the rate of the reaction and the productivity of the catalyst system drop to a low level. As known from the article in Applied Catalysis A: General, vol. 193 (2000), pp. 29-38, the rate of the trimerization reaction for the chromium-based catalyst system is proportional to the square of ethylene pressure. Thus, in decreasing the pressure from 37.4 bar to 12 bar, the catalytic activity decreases in about 10 times, and, in decreasing the pressure to 8 bar, it decreases in 21 times, i.e., to about 400 g/(g Al·hr). The above patent discloses various embodiments of preparation of catalyst systems for trimerization, oligomerization, and polymerization processes. Disadvantages of the prototype are the necessity of using expensive germanium tetrachloride as a halide source to achieve high activity and selectivity of the system, and considerable decrease in the activity of the catalyst system when decreasing the pressure. Therefore, it is necessary to use increased pressure of ethylene, which, in turn, leads to high capital expenditures for the equipment.